Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Fri Aug 11 2023 Valid 12Z Mon Aug 14 2023 - 12Z Fri Aug 18 2023 ...Severe weather and flash flooding possible across parts of the Upper Midwest to Eastern U.S. early next week... ...Heat wave to prevail from the Central Gulf Coast westward into the Southwest... ...Hazardous heat across the Pacific Northwest into mid next week... ...Overview... Ridging is most persistent across the Southwest this period as a trough tries to take over the flow pattern as much as is feasible in mid to late August. The western and central Gulf Coast will be mostly left out of the troughing aloft this period as a front tries to near the region while the Northwest slowly backs out off heatwise as warm core ridging relents and fronts attempt to make progress into the area. The first system moving through the base of the trough should be the most robust, moving from the Upper Midwest, across the Great Lakes, and into the Northeast the first half of next week, possibly generating episodes of strong to severe convection and heavy rainfall. ...Guidance Evaluation/Predictability Assessment... Outside of the 00z Canadian, the other guidance was reasonably agreeable. This led to a equal blend of the 00z UKMET, 00z Canadian, 00z GFS, and 00z ECMWF for Monday into Tuesday, before slowly replacing the Canadian/UKMET with the 00z NAEFS/ECMWF ensemble means for the blend used in the pressures, QPF, 500 hPa heights, and winds. The 13z NBM was the basis for the rest of the grids, with some adjustment in the PoPs made for the QPF. ...Weather/Hazards Highlights... The most prominent focus for active weather during the period will be from the Midwest/Great Lakes into the Northeast next Monday-Tuesday. Strong cold front sweeping through the region should support areas of strong to severe thunderstorms and heavy rainfall along and ahead of the boundary. This is reflected in the Day 4 WPC Excessive Rainfall Outlook highlighting slight risks near the Great Lakes/Michigan, where local 2-4" amounts are expected near and along the track of the surface low, with a second risk area across the central Appalachians which has similar rainfall potential. Both saw some changes in the afternoon update. The Storm Prediction Center also highlights a risk for severe thunderstorms across parts of the Ohio Valley/central Appalachians on Monday. This system should race quickly through the Northeast on Tuesday, but given anomalous moisture and sensitivity in some regions of the Northeast, heavy rainfall and/or flash flooding cannot be ruled out. A marginal risk remains across this region to the Day 5 excessive rainfall outlook, though it is on the low end of the Marginal Risk as it appears that the guidance is showing amounts which are lighter. Some risk for heavy rains near an area of low pressure along the boundary for eastern North Carolina as well on Tuesday and the lingering portion of the boundary near the Gulf Coast. The broad troughing across the Great Lakes should continue unsettled weather into the latter part of next week for this region. Elsewhere, monsoonal moisture will continue into the Southwest/Four Corners next week, peaking Monday into Tuesday. As such, marginal risks were carried on both days in the Excessive Rainfall Outlooks, with only small modifications in the afternoon update. Oppressive heat will continue through at least next week from the central Gulf Coast westward. It will be most persistent from southern and southeast Texas into Louisiana, with some moderation expected across the Southeast/Florida mid to late next week. Numerous daily record highs and warm overnight lows are likely. The combination of heat and humidity will also result in extremely dangerous heat indices in excess of 110F in many locations. Heat should also build back into the Southwest later next week as well. Meanwhile, a second area of heat is expected to build into parts of the Northwest this weekend and through mid next week, with daytime highs 10 to 20 degrees above normal through Thursday. High temperatures in the 90s, 100s, and even localized 110s possible are expected across Washington, Oregon, and the interior Valley of California; Oregon should see the most persistent heat. Erosion of the warm core ridge across the Northwest with time should begin to moderate temperatures on Thursday, though more substantially by next Friday. See key messages issued by WPC for additional information and graphics related to these two heat waves. Roth/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw